1. Field of the Invention
The present invention relates to an outline compensation apparatus for providing a controlled degree of enhancement of boundaries between regions of differing brightness in a picture displayed by a television receiver.
2. Description of Related Art
Recently developed types of television receivers having a large-screen display are generally provided with circuits for effecting a controlled degree of enhancement of the boundaries between adjacent regions which are of mutually different brightness. Such circuits of a television receiver will be collectively referred to in the following as an outline compensation apparatus. In the case of a television receiver having a cathode ray tube (hereinafter referred to as CRT) display, outline enhancement is usually executed by a velocity modulation circuit, which varies the horizontal scanning velocity of the electron beam of the CRT such as to make the transitions between different regions of the displayed picture become more abrupt, to thereby apply compensation for various signal processing effects which tend to cause the transitions to become gradual. Such outline enhancement (sometimes referred to as "edge enhancement") results in an apparent increase in sharpness of definition of these boundaries, as seen by the viewer. To control such a velocity modulation circuit, it is necessary to detect the outlines, i.e. boundaries between regions of different brightness, by detecting sudden changes in amplitude of the luminance signal component of the television signal. This is usually achieved by amplifying and then differentiating the luminance signal, at the point in the television signal path where the chroma signal and luminance signal are separated from the composite video signal (e.g. by means of a comb filter). In parallel with that outline detection processing, the luminance signal is subjected to various processing to modify the level of contrast and degree of sharpness of definition of the picture, before being finally applied to drive the cathode of the CRT. Thus, the luminance signal that is used for the purposes of outline detection is essentially different from the luminance signal which is applied to the cathode of the CRT.
As a result, with such a prior art outline compensation apparatus, if for example the user adjusts the picture contrast control such as to lower the degree of contrast, then there will be a resultant change in the luminance signal that is applied to the cathode of the CRT, but there will be no change in the luminance signal that is supplied to the outline detection circuit. Hence the outline compensation apparatus will continue to apply the same amount of outline enhancement. Since a relatively high degree of outline enhancement contributes to an increased degree of (visually perceived) display contrast, the operation of the outline compensation apparatus in that case will act to reduce the effectiveness of the reduction of contrast by the user. Similarly, if the user adjusts the picture sharpness control (i.e. to thereby adjust the level of certain high-frequency components of the luminance signal that is applied to the cathode of the CRT, thereby varying the sharpness of definition of detail in the displayed picture) such as to lower the degree of sharpness of definition, there will be no change in the luminance signal that is supplied to the outline detection circuit. Hence the outline compensation apparatus will continue to apply the same amount of outline enhancement. Since a relatively high degree of outline enhancement contributes to an increased degree of (visually perceived) overall sharpness of definition in the displayed picture, the operation of the outline compensation apparatus in that case will again act to reduce the effectiveness of the reduction of contrast that is desired by the user.
Thus, when the user attempts to adjust the contrast and/or sharpness of the displayed picture in order to provide a more natural-appearing picture, the effectiveness of such adjustment of the picture quality will be reduced due to the operation of the outline compensation apparatus.
It would be possible to overcome this problem by inputting to the outline detection circuit of the outline compensation apparatus the luminance signal after that signal has been subjected to contrast adjustment and sharpness adjustment. However in that case the input signal to the outline detection circuit will be at a position (i.e. along the circuit path extending from the point of separation of the chroma and luminance signals to the point of connection to the CRT cathode) that is very close to the CRT. Hence the delay time between the time at which the outline compensation apparatus receives the luminance signal and the time at which the signal reaches the cathode of the CRT will be very short. It therefore becomes necessary to carefully match the phase of the waveform of the drive current in the velocity modulation coil to the waveform of the signal that is applied to the CRT cathode. Such phase matching will be required for both luminance and chroma components, and will result in increased manufacturing cost, due to the need to incorporate suitable delay elements.